Attempts to pultrude a fiber reinforced plastic (FRP) skin onto a preformed light density foam core board have been largely ineffective. One of the major problems of such attempts has been the friability of the foam material when subjected to the pressure of the pultrusion process. Without a significant amount of pressure, the FRP skin will be cosmetically unacceptable.
One method has been developed for manufacturing a composite sandwich structure by pultrusion of a skin of liquid resin and reinforcement materials on the surface of a preformed foam core. As the liquid resin and foam core are pulled through the pultrusion die, the liquid resin and the surface region of the foam core are heated to convert water in the foam core to steam, thereby causing water vapor pressure expansion of the foam core. The pultrusion die constrains the liquid resin so that the expansion of the foam core subjects the liquid resin to increased pressure, thereby forcing the liquid resin to penetrate and wet the reinforcement materials and to partially penetrate the foam core. See U.S. Pat. No. 5,286,320.
The use of foam panels in the pultrusion process still requires preformed boards. Typically, foam board is purchased in 4'.times.8' sheets, trimmed to size and fed into the pultrusion process. This method either limits the pultrusion process to simple rectangular shapes of low cost or destroys the favorable economics with a foam shaping operation with subsequent foam scrap. This process also requires, in the pultrusion process, temperatures of at least 100.degree. C. and excessive water vapor expansion pressures. As a result, this pultrusion process is limited to producing hollow shapes with wall thicknesses of 0.070" or greater.